Constructional Design of Compounding Plant

Machinery used to prepare particulate-filled polymer composites may be classified in terms of the applied shear intensity and whether the operation is of a batch or continuous nature. Furthermore, most compounding processes are critically dependent on the efficiency of ancillary equipment to undertake various functions, including additive and polymer feeding, melt filtration and pelletising procedures. 

With this wide range of material types and approaches to compounding, incorporation of particulate additives into polymers can vary greatly from one compoimd to another. Attention will therefore be directed only on the main types of compoimding plant used industrially, highlighting their principles of design and operation specifically for the preparation of particulate-filled polymeric compositions. 

Frequently the compounding process entails preliminary combination of polymer and additive(s) through an initial blending stage, to pre-distribute the formulation, prior to high shear mixing with polymer in a molten state. 

During premixing, the components are randomly interspersed with each other and levels of developed shear are generally low. Formulations include solid/solid mixtures of polymer and additive particles, where temperatures are maintained below the polymer melting point, or low-viscosity liquid/solid formulations, as is found with PVC pastes and DMC premixes. 

Conical screw mixers also contain a screw for conveying and mixing, but this is located inside an inverted conical-shaped mixing chamber. The screw is angled from the base of the mixer to the outside of the top. It may revolve on its own axis at about 60 rpm, whilst slowly orbiting the cone at around 3 rpm. Pelletised or powdered polymer, and additives are transferred from bottom to top by the rotating screw, from the walls to the centre of the chamber as the screw circulates, to fall on to material being carried upwards. 

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